of stockholm



Patented Mar. 8, 1932.

UNITED STATES PATENT OFFICE GUSTAF EAGLUND, OF STOCKHOLM, SWEDEN,ASSIGNGR '1"O PATENTAKT'IEBOLAGEI." GBfiNDAL-RAMEN, OF STOCKHOLM, SWEDENS'U'LPHITE CELLULOSE DIGESTION No Drawing. Application filed March 31,1931, Serial No. 526,762, and in Germany April 15, .1980.

In the usual sulphite cellulose digestion where a solution containingbi-sulphite of lime or alkali and a surplus of sulphur diox ide over andabove that required to combine with normal sulphite to form bi-sulphiteis used as cooking liquor, which surplus the following is called freesulphur dloxide, and where'the digestion is made under pressure at atemperature of 120? C. or above, it has appeared that the free sulphurdioxide content of the liquor rapidly diminishes even at the beginningof the digestion and before the chips that are to be digested have beenthe chips at softened or their lignin substances dissolved.

This isdue to the fact that the pores of the chips "(gas and aircavities) get filled with sulphur dioxide, because the free sulphurdioxide escapes from the digesting liquor even at temperatures below theboiling. point of the liquor and consequently enters into the chipsbefore these have been penef trated'by the liquor. Lignin and othernoncellulose components that are to be dissolved from the chips, have astrong inclination to take up sulphur dioxide at an early stage of thedigestion and forsaid reason there is a strong concentration of sulphurdioxide in dioxide content of the liquor at the beginning of thedigestion and before the liquor has completely penetrated the chips. p

This rapid entry of sulphur dioxide into the beginning of the digestionhas an unfavorable influence on the further progress of the digestionbecause the sulphur dioxide collected in the pores of the chips is ahindrance to thepenetration of the .liquor into the chips and alsobecause the concentration of sulphur dioxide in the chips eifects a toora id reaction with the. lignin content of the chips whereby ligninsulphonic acids diflicult to dissolve and of an unstable character areformed.

The bi-sulphites of the liquor should react with the lignin-sulphonicacids formed through the sulphur dioxide reaction so as to formlignin-sulphonic salts whichsalts are more easily dissolved and morestable than the sulphonic acids primarily formed and it results fromthis that if the liquor cannot diminishing of the sulphur penetrate intothe chips freely or if the sulphur dioxide reaction in the chips takesplace too rapidly before the penetration of the liquor the consequencesas regards the digesportions of the chips, dark coloring of thecellulose and an incomplete digestion.

To prevent these lnconveniences, in the usual sulpliitedigestionprocess, care is taken when starting the digestion to have thetemperature rise slowly which causes a slackening up of the reactionspeed of sulphur dioxide, providing at the same time an opportun-- ity'for the liquor to enter with its contents of bi-sulphite into the chips.

If the digestion is carried out with alkalibi-sulphite, therecovery ofthe alkali is facilitated by keeping the waste liquors at a high degreeof concentration and for that purpose varying quantities of wast-eliquors from previous digestions are added to that c'oooking liquor.WVhen using such a liquor in the digestion another inconvenience willpresent itself, namely that the liquor penetrating into, the chipscarries with it large quantities of organic materials which are easilyinfluenced by the sulphur dioxide concentrated in the chips in the sameway as the organic contents of the chips themselves. It has consequentlyproven to be a diflicult thing to carry out digestions of sulphitecellulose with cooking liquors containing waste liquors from previousdigestions.

The. present invention aims to improve the digestion process by removingthe causes of an irregular digestion process, and also to.

render it possible to addwaste liquors to the liquor used for digestionof sulphite cellulose.

In the usual digestion processes where bisulphite and sulphur dioxideare j0intly introduced into the digester in' the same liquor,

' bility factors, it is preferable to use alkali bi-sulphite in thesecases. The second solution is prepared by introducin sulphur dioxide gasfrom a digester or from another source preferably into waste liquors orweak liquors obtained when washing cellulose from earlier digestions,and introducing it in such a quantity that the solution will have a highcontent of free sulphur dioxide or is saturated with sulphur. dioxide atthe temperature of the solution. A

This solution contains the free sulphur dioxide that is required for thereaction and the presence of bi-sulphite in this solution is immaterial.

' The above two solutions form the real digestion liquor but they arenot introduced simultaneously.

The process may be suitably carried out in the following way: To thedigester which is filled with chips the bi-sulphite solution is added.After the chips have been saturated with the solution the major part ofit is tapped oil so that only a smallquantity remains.

After this, the digester is filled up with the solution prepared fromwaste liquor or washing water containing a high concentration of freesulphur dioxide, and the digestion is started.

As the temperature of the digester rises the sulphur dioxide gas is setfree in the usual way, but it is prevented from penetrating too rapidlyinto the chips, for the same are already saturated with bi-sulphitesolution and no abnormal concentration of sulphur dioxide gas ispossible which would give rise to a premature influence on the ligninsubstances by the sulphur dioxide.

The bi-sulphite of the solution with which the chips are saturatedbefore the sulphur dioxide reactions can occur, immediately reacts withthe lignin-sulphonic acid formed by the penetrating sulphur dioxide, andthe reactions will therefore take a regular and normal course even whenno precautions are taken as regards a rapid rise of the temperature whenstarting the digestion.

As dry chips can take up their own weight of bi-sulphite liquor, thealkali bi-sulphite used for the preliminary soaking of the chips, may beprovided with such a high concentration of bi-sulphite that before theaddition of S0 solution, the chips contain'the quantity of bi-sulphiterequired for carrying out the digestion process.

The solution of sulphite added to the digester may be made soconcentrated that for instance a quantity of solution corresponding toone fourth only of the free Volume of the digester will contain enoughalkali metal sulphites for carrying through the reactions noted. Inusing such a solution, the quantity introduced into the digester mayeither correspond approximately 'to one fourth of the free volume of thedigester, in which case there will be no subsequent removal of thesolution, orif desired the digester may be filled more or lesscompletely with such solution. In the latter case, however, after thesoaking with the concentrated sulphite solution has been completed, theexcess solution should be tapped off from the digester, at least to theextent that sufficient space .will be provided for the succeedingsolution of sulphur dioxide, and to a further extent if desired.

While in the foregoing I have indicated the use of a solution of sulphurdioxide in waste liquor and/or washing water from previous digestions,and in practice it is more expedient to use such a carrier for thesulphur dioxide, the solution may if desired consist simply of anordinary water solution of the sulphur dioxide without any complement ofwaste liquor and/or washing water.

I claim as my invention:

1. The sulphite cellulose -digestion process which comprises, firstintimately contacting the cellulose-containing material with-a solutionof a sulphurous acid salt of the group: alkali metal, alkaline earth,then adding a solution of sulphur dioxide, and digesting thecellulosecontaining material in said solutions.

2. The sulphite cellulose digestion process which comprises, firstintimately contacting the cellulose-containing material with a solutionof an alkali metal salt of sulphurous acid, then adding a solution ofsulphur dioxide, and digesting the cellulose-containing material in saidsolutions.

3. The sulphite cellulose digestion process which comprises, firstintimately contacting the cellulose-containing material with a solutionof an alkaline earth salt of sulphurous acid, then adding a solution ofsulphur dioxide, and digesting. the cellulose-containing material insaid solutions.

4. The sulphite cellulose digestion process which comprises, firstintimately contacting the cellulose-containing material with a so lutionof an alkali metal salt of sulphurous acid, then adding a solution ofsulphur dioxide in waste liquorand/or wash water from a similardigesting operation to obtain a'h1gher concentration of organic matrter' 1n the waste liquor of the instant operation, and digesting thecellulose-containing material in said solutions.

5'. The sulphite cellulose digestion process which comprises, firstsoaking the cellulosecontaining material in a solution of a sulphurousacid salt of the group: alkali metal, alkaline earth, until themateriali's suitably saturated with said solution, then adding asolution of sulphur dioxide, and digesting the cellulose-containingmaterial in said solutions. i

6. The sulphite cellulose digestion process which comprises, firstintimately contacting the cellulose-containing material with a solutionof a sulphurous acid salt of the group: alkali metal, alkaline earth,said solution being provided in excess of that capable of being absorbedby said material, removing a substantial portion of the excess solutionafter absorption, then adding a solution of sulphur dioxide, anddigesting the cellulosecontaining material in said solutions.

7. The sulphite cellulose digestion process which comprises, firstintimately contacting the cellulose-containing material in a solution ofa sulphurous acid salt of the group: alkali metal, alkaline earth, saidsolution having such concentration of the sulphurous acid salt that thequantity of solution absorbed by said material will contain suflicientsulphurous acid salt for carrying out digestion, then adding a solutionof sulphur dioxide, and digesting the cellulose-containing material insaid solutions.

8. The sulphite cellulose digestion process which comprises, firstsoaking the cellulosecontaining material in a solution of the group:alkali metal, alkaline earth, bi-sul-.

phite, then adding a solution of sulphur dioxide, and digesting thecellulose-containing material in said solutions.

9. The sulphite cellulose digestion process 7 which comprises, firstsoaking the cellulosecontaining material in a solution of alkali metalbi-sulphite until said material has substantially reached its 'abso tionlimit as regards said solution, said i-sulphite being provided in suchconcentration in said solution that the quantity of bisulphite convtained in the solution absorbed is sufiicient for carrying out thedigestion, then adding a solution of sulphiir dioxide, and digesting thecellulose-containing material in said solutions.

10. The sulphite cellulose digestion process which comprlses, firstsoaking the cellulosecontaining material with a solution of a sulphurousacid salt of the group: alkali metal,

alkaline earth in a di ester, removing from the digester such portlon ofthe. unabsorbed solution as .is necessary to accommodate the quantity'ofa sulphur dioxide solution to be used for the digestion process, saidfirstnamed solution containing said salt in such concentration that theportion remainingin said digester provides a sufiicient quantity

